The present invention relates to a novel polypropylene resin composition or, more particularly, to a polypropylene resin composition-capable of giving a polypropylene resin film having excellent resistance against blocking or slipperiness of the surface so as to be smoothly wound up in a roll in the film-shaping works and to be suitable as a wrapping material with respect to the ease of opening the mouth of a film bag prepared therefrom.
One of the important applications of a polypropylene resin is as a material of resin films for wrapping use. Polypropylene resins in general, however, have a serious drawback as a material of resin films because polypropylene resins usually have tackiness on the surface so that the winding velocity of the resin film as shaped into a roll is greatly limited thereby to cause blocking resulting in a decrease in the productivity of the resin films. This is because the velocity of rewinding of a rolled film cannot be high enough, for example, in the bag-making works and thereby greatly decrease the working efficiency. Furthermore, film bags prepared from a polypropylene resin film are not always quite convenient in use because the resin films at the folded bag mouth sometimes firmly adhere to each other causing difficulty when the bag mouth is hastily opened by pulling the resin films apart.
Several attempts and proposals of course have been made heretofore in order to solve the above mentioned problem due to the surface tackiness of polypropylene resin films, mainly, by incorporating various kinds of anti-blocking agents into a polypropylene resin to give a resin composition. The anti-blocking agents heretofore proposed include, for example, anhydrous active silicic acid disclosed in Japanese Patent Publication No. 39-16908, kaolinite disclosed in Japanese Patent Kokai No. 43-24523, finely pulverized zeolite disclosed in Japanese Patent Publication No. 52-16134 and other inorganic powdery materials. These anti-blocking agents are of course not ineffective but effective at least to some extent in exhibiting an anti-blocking effect when the resin composition is shaped into a resin film. When a high anti-blocking effect is desired with these anti-blocking agents, however, the compounding amount of the anti-blocking agent must be so large that the polypropylene resin film shaped from the resin composition has a greatly decreased transparency so that the resin film can be used in a limited field of applications. In this regard, proposals have been made that the decrease in the transparency of the resin films can be mitigated when the resin film is shaped from a polypropylene resin composition compounded with silicon dioxide and a magnesium salt or aluminum salt of a higher fatty acid in combination as disclosed in Japanese Patent Kokai No. 58-225142 or with an inorganic powder of which the particles have a lamellar structure disclosed in Japanese Patent Kokai No. 57-18747 although these methods do not provide a full solution of the problem.
Furthermore, a silicone resin powder is proposed as an antiblocking agent in a crystalline polypropylene resin film (see Japanese Patent Kokai No. 62-215646) or a resin film of a propylene-.alpha.-olefin copolymer (see Japanese Patent Kokai No. 62-232448 and No. 62-233248). This method is, indeed, effective to some extent in solving the above described problems but the effectiveness cannot be high enough unless the amount of the silicone resin powder compounded in the resin composition is so large as to cause an economical disadvantage due to the great increase in the cost of the resin composition.